"Whoever" is correct. The indirect object of the verb in the main clause is the entire noun clause, not just the head of the clause.
Directly quoted from "Cliff's notes" (sorry for the long inset)Pronoun case in subordinate clause
Who, whom, whoever, whomever. In deciding which case of who you should use in a clause, remember this important rule: The case of the pronoun is governed by the role it plays in its own clause, not by its relation to the rest of the sentence. Choosing the right case of pronoun can be especially confusing because the pronoun may appear to have more than one function. Look at the following sentence.
At first, you may be tempted to think whomever rather than whoever should be the pronoun here, on the assumption that it is the object of the preposition to. But in fact the entire clause, not whoever, is the object of the preposition. Refer to the basic rule: The case should be based on the pronoun's role within its own clause. In this clause, whoever is the subject of the verbpresented. (A good way to determine the right pronoun case is to forget everything but the clause itself: whoever presented the winning ticket, yes; whomever presented the winning ticket,no.)
The following two sentences show more dramatically how you must focus on the clause rather than the complete sentence in choosing the right pronoun case.
In each sentence the clause is the direct object of asked. But in the first sentence, whomever is correct because within its clause it is the object of saw, while in the second sentence, whoeveris correct because it is the subject of called.